Why Did Great Basin Eocene Magmatism Generate Carlin-type Gold Deposits When Extensive Jurassic to Middle Miocene Magmatism Did Not? Lessons from the Cortez Region, Northern Nevada, USA

Most major precious and base metal deposits in the Great Basin formed in the
Eocene, including porphyry systems (e.g., Battle Mountain, Bingham Canyon, and
Mt Hope) and Carlin-type Au deposits (CTDs). A favored source of Au in CTDs is
Eocene magma, which could have acquired Au directly from the mantle or through
crustal assimilation. Yet Jurassic, Cretaceous, and Eocene magmatism in northern
Nevada was similarly arc-related and transected the same crust. Unless different
magmatic episodes interacted with different crustal levels, why are no CTDs associated with older magmatism? Or are they?—several studies propose that CTDs are
composites of Eocene and Cretaceous mineralization or that Au was recycled from
Paleozoic sedimentary exhalative (sedex) concentrations in the Eocene.
The Cortez—northern Shoshone Range region exemplifies these uncertainties.
Extensive 40Ar/39Ar geo- and thermochronology and U-Pb zircon dates show the region underwent major intrusive episodes in the Jurassic (160–162 Ma), Cretaceous
(104–114 Ma), Eocene (two periods at 39–40 and 34–36 Ma), and Miocene (~16 Ma),
all with concurrent hydrothermal activity. Jurassic rocks include the Mill Canyon
quartz monzonite—granodiorite and porphyritic rhyolite and lamprophyre dikes.
The only exposed Cretaceous rock is a dacite dike in Mill Canyon, but drilling has
intersected Cretaceous granodiorite at Gold Acres (105 Ma) and in two locations near
Cortez Hills (104 and 109 Ma). Zircon xenocrysts in Eocene rocks indicate additional
Cretaceous intrusions in the subsurface. 39–40 Ma granodiorite plutons and dikes are
prominent in the northern Shoshone Range at Hilltop, Granite Mountain, and Tenabo. All Jurassic, Cretaceous, and 39–40 Ma plutons were shallowly emplaced (mostly ≤ ~4 km). 34–36 Ma activity includes 35.7 and 36.3 Ma porphyritic rhyolite dikes focused around Cortez and forming a band from Tenabo to the northern Simpson Park
Mountains, scattered andesite—dacite lavas and minor tuffs, and the voluminous 34
Ma Caetano Tuff and intrusions. Miocene rocks are extensive basaltic andesite lavas
and dikes and one rhyolite lava dome of the northern Nevada rift.
Jurassic mineralization consists of Mo in stockwork-veined rhyolites and some
Ag±Au+base metal veins (Empire State Mine) in the Mill Canyon intrusion. The Cortez Ag district and at least one Ag-base metal replacement deposit (Berlin Mine) in
Mill Canyon are Cretaceous based on 40Ar/39Ar sericite dates. The 39–40 Ma, northern Shoshone Range granodiorites generated minor to significant porphyry-related
mineralization but no definite CTDs.
Only Cortez Hills of the major Cortez area CTDs is well dated at 35.7 Ma, based
on rhyolite dikes ranging from strongly mineralized to unaltered in the deposit. An
underlying pluton—not its shallow dikes—was the heat source for mineralization.
A published interpretation is that Pipeline is partly Cretaceous, and Cretaceous hydrothermal systems are spatially and temporally related to the adjacent Gold Acres
intrusion, but the age of Pipeline CTD is unresolved. The age of the recent GoldrushFourmile discovery is unknown. A key question is whether Pipeline and GoldrushFourmile also formed at 35.7 Ma.
Superposition of Mesozoic and Eocene igneous and hydrothermal episodes hampers resolving which igneous episode generated what mineralization and whether older events contributed to Eocene CTDs, both around Cortez and throughout northern
Nevada. Nevertheless, we find no evidence for contributions to any Cortez-area CTDs
from Jurassic, Cretaceous, or 39–40 Ma mineralization (most of which is Ag-base
metal rich), nor from recycling of Paleozoic sedex concentrations. Although all ages
of igneous-hydrothermal episodes are present in many areas of CTDs, no CTDs are
demonstrably other than Eocene.
One lesson from the Cortez region is that lack of exposed intrusive rocks of any
age does not preclude their presence even in the shallow subsurface. At Cortez these
“hidden” intrusions are Cretaceous, but Eocene and other intrusions could also be
present in places where they are not yet known. Two ways to identify hidden Eocene
intrusions are analyzing younger igneous rocks for zircon xenocrysts and 40Ar/39Ar
thermochronology that can test for heating of older rocks by Eocene magmatism.
The possible restriction of CTDs to the 35.7 Ma magmatic episode in the Cortez
region leads to two speculative contributions to the numerous factors that have been
interpreted to influence CTD formation. CTDs may be associated only with deeply
emplaced (6–10 km), high-SiO2 Eocene plutons. In contrast, shallowly emplaced Eocene intrusions are associated with mineralization related to porphyry Cu systems,
including Cu-Au skarn, polymetallic replacement and vein, and distal disseminated
Au deposits, and lack or have only small CTDs.
Key Words: Cortez, mineralization, Carlin-type deposits, magmatism, Eocene, Cretaceous